A GSM phone includes a subscriber identity module (SIM) card, which has an international mobile subscriber identity (IMSI) stored thereon. An IMSI is a unique 64-bit number. Within a GSM network, the IMSI is associated with the MSISDN, sometimes referred to as the “Mobile Station International Subscriber Directory Number,” or colloquially as the telephone number. The MSISDN is a unique number that identifies a subscription on the GSM network and is also the number that can be used to place a call to the GSM phone. The MSISDN is not necessarily stored on the SIM card or the phone, and the MSISDN is not permanently associated with the IMSI. The MSISDN may change if the subscription changes, for example.
In a typical GSM phone registration situation, the phone registers with a mobile switching center (MSC), and a visitor location register (VLR) is updated. After the registration authentication procedure is completed, the MSC/VLR sends a location update mobile application part (MAP) message to a home location register (HLR) with the IMSI of the phone that is registered. The HLR replies back with a MAP insert subscriber (ISD) message that provides subscription information to the MSC/VLR including the MSISDN.
When a call comes into the GSM network, it passes through a gateway MSC, which sends out a send routing information (SRI) message to the HLR that the phone is registered with. The HLR sends a provide roaming number (PRN) message to the MSC where the user device is registered, the MSC sends back a mobile station roaming number (MSRN), and the HLR sends the MSRN as a response to the SRI. The MSRN is a temporary number that the MSC/VLR maps to the MSISDN. The gateway MSC routes the call to the MSC that the called party is registered on using the MSRN.
To enable multi-line services—such as two phone numbers for one mobile phone—a dual SIM phone may be used. The SIM stores two IMSIs, and registers one or both of them with the MSC/VLR, depending on the mode of the phone.